Signal transmission cable, multicore cable, and method of manufacturing signal transmission cable

ABSTRACT

A signal transmission cable includes a signal line, an insulation layer configured to cover the signal line, and a plating layer configured to cover the insulation layer. An arithmetic average roughness Ra of an outer peripheral surface of the insulation layer is between 0.6 μm and 10 μm inclusive. A method of manufacturing the signal transmission cable includes covering the signal line with the insulation layer, followed by conducting a dry-ice-blasting on the outer peripheral surface of the insulation layer, followed by conducting a corona discharge exposure process on the outer peripheral surface, and forming the plating layer on the outer peripheral surface.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of Japanese Patent Application No.2017-131094 filed Jul. 4, 2017 in the Japan Patent Office, thedisclosure of which is incorporated herein by reference.

BACKGROUND

The present disclosure relates to a signal transmission cable, amulticore cable, and a method of manufacturing the signal transmissioncable.

Signal transmission cables are conventionally used for signaltransmissions between electronic devices, or signal transmissionsbetween substrates in electronic devices. Signal transmission cablecomprises, for example, a signal line, an insulation layer that coversthe signal line, and an external conductor that covers the insulationlayer as disclosed in JP 2002-289047 (Patent Document 1). The externalconductor is formed by, for example, winding a metallic tape or ametallic braid around an outer circumference of the insulation layer.

SUMMARY

There has been a problem with the conventional signal transmission cablethat a gap is occassionally created between the insulation layer and theexternal conductor. Such a gap may reduce a shielding effect by theexternal conductor to an unsatisfactory level.

Aspects of the present disclosure provide a signal transmission cable, amulticore cable, and a method of manufacturing the signal transmissioncable that can reduce a gap between an insulation layer and an externalconductor.

One mode of the present disclosure is a signal transmission cable thatcomprises a signal line, an insulation layer configured to cover thesignal line, and a plating layer configured to cover the insulationlayer. An arithmetic average roughness Ra of an outer peripheral surfaceof the insulation layer is between 0.6 μm and 10 μm inclusive.

In a signal transmission cable according to a first aspect of thepresent disclosure, the arithmetic average roughness Ra of the outerperipheral surface of the insulation layer is 0.6 μm or greater. Thisresults in high adhesion between the insulation layer and the platinglayer, which then reduces occurrence of a gap between the insulationlayer and the plating layer. As a consequence, a high shielding effectby the plating layer is provided. At the same time, transmission losscan be reduced since the arithmetic average roughness Ra of the outerperipheral surface of the insulation layer is 10 μm or less.

A second aspect of the present disclosure provides a multicore cablecomprising signal transmission cables, a conductor layer configured tocover the signal transmission cables collectively, and a jacketconfigured to cover the conductor layer. Each of the signal transmissioncables in the multicore cable comprises the signal transmission cableaccording to the first aspect of the present disclosure, and an outerinsulation layer configured to cover the plating layer.

In the signal transmission cables of the multicore cable according tothe second aspect of the present disclosure, the arithmetic averageroughness Ra of the outer peripheral surface of the insulation layer is0.6 μm or greater. This results in high adhesion between the insulationlayer and the plating layer, which then reduces occurrence of a gapbetween the insulation layer and the plating layer. As a consequence, ahigh shielding effect by the plating layer is provided. At the sametime, transmission loss can be reduced since the arithmetic averageroughness Ra of the outer peripheral surface of the insulation layer is10 μm or less.

A third aspect of the present disclosure provides a method ofmanufacturing the signal transmission cable comprising a signal line, aninsulation layer configured to cover the signal line, and a platinglayer configured to cover the insulation layer. The method comprisesapplying a dry-ice-blasting to an outer peripheral surface of theinsulation layer, then applying a corona discharge exposure process tothe outer peripheral surface, and then forming the plating layer on theouter peripheral surface.

With the method of manufacturing the signal transmission cable accordingto the third aspect of the present disclosure, the arithmetic averageroughness Ra of the outer peripheral surface of the insulation layer canbe increased to enhance the surface-wettability. Accordingly, amanufactured signal transmission cable has high adhesion between theinsulation layer and the plating layer, which reduces occurrence of agap between the insulation layer and the plating layer. As aconsequence, a high shielding effect by the plating layer is provided.

BRIEF DESCRIPTION OF THE DRAWINGS

Example embodiments of the present disclosure will be describedhereinafter with reference to the accompanying drawings, in which:

FIG. 1 is a perspective view showing a configuration of a signaltransmission cable 1;

FIG. 2A is a micrograph showing a result of an adhesion test of acomparative example;

FIG. 2B is a micrograph showing a result of an adhesion test of a firstembodiment;

FIG. 3A is a micrograph showing a state of a surface of a copper platinglayer formed on the comparative example;

FIG. 3B is a micrograph showing a state of a surface of a copper platinglayer formed on a reference example;

FIG. 3C is a micrograph showing a state of a surface of a copper platinglayer formed after dry-ice-blasting and corona discharge exposure;

FIG. 4 is a graph showing a correlation between an arithmetic averageroughness Ra and a contact angle of a polyethylene substrate afterspecific surface modification treatment;

FIG. 5 is a graph showing a correlation between an arithmetic averageroughness Ra and an adhesion-wetting surface free energy of apolyethylene substrate after specific surface modification treatment;

FIG. 6 is a micrograph showing a concavity formed on an outer peripheralsurface of an insulation layer after dry-ice-blasting;

FIG. 7 is an X-ray diffraction pattern of an insulation layer specimen;

FIG. 8 is an X-ray diffraction pattern of an insulation layer specimen;

FIG. 9A is a graph showing a correlation between a crystallinity X_(c)and a contact angle of a polyethylene substrate after specific surfacemodification treatment;

FIG. 9B is a graph showing a correlation between a crystallinity X_(c)and a contact angle of a perfluoroethylene-propene copolymer substrateafter specific surface modification treatment;

FIG. 10 is an X-ray diffraction pattern of an insulation layer specimen;

FIG. 11 is an X-ray diffraction pattern of an insulation layer specimen;

FIG. 12 is a graph showing a correlation between a (100) crystalorientation degree O₁₀₀ and a contact angle after specific surfacemodification treatment;

FIG. 13A is a graph showing a correlation between a crystalline size Dand a contact angle of a crystalline component of polyethylene afterspecific surface modification treatment;

FIG. 13B is a graph showing a correlation between a crystalline size Dand a contact angle of a crystalline component ofperfluoroethylene-propene copolymer after specific surface modificationtreatment;

FIG. 14 is an explanatory diagram showing a configuration of amanufacturing system 101;

FIG. 15 is an explanatory diagram showing a configuration of amanufacturing system 201;

FIG. 16 is an explanatory diagram showing a configuration of a surfaceimproving unit 203;

FIG. 17 is an explanatory diagram showing a configuration of the surfaceimproving unit 203′;

FIG. 18 is an explanatory diagram showing a configuration of the surfaceimproving unit 203″;

FIG. 19 is a sectional view showing a configuration of a multicore cable301;

FIG. 20 is a sectional view showing a configuration of a differentialsignal transmission cable 302 included in the multicore cable 301;

FIG. 21 is a graph showing a differential-to-common mode conversionenergy of a differential signal transmission cable 1 and a comparativeexample;

FIG. 22 is a graph showing a transmission loss of the differentialsignal transmission cable 1 and the comparative example; and

FIG. 23 is a graph showing a relation between an arithmetic averageroughness Ra and a transmission loss Sdd21.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, embodiments of the present disclosure will be explained.

1. Signal Transmission Cable

(1-1) Basic Configuration of Signal Transmission Cable

A signal transmission cable in the present disclosure comprises a signalline, an insulation layer configured to cover the signal line, and aplating layer configured to cover the insulation layer. The signaltransmission cable in the present disclosure may be, for example, adifferential signal transmission cable or other types of signaltransmission cables. A differential signal transmission cable comprisestwo signal lines.

In a case where the signal transmission cable in the present disclosureis provided as a differential signal transmission cable, a signal can betransmitted to a receiving device by a differential signal. In thesignal transmission using the differential signal, signals having phasesopposite to each other are input to the two signal lines respectively.The receiving device synthesizes the difference between the two signalshaving the opposite phases to obtain an output.

The signal transmission cable in the present disclosure has, forexample, a configuration shown in FIG. 1. The example of the signaltransmission cable shown in FIG. 1 is an example of the differentialsignal transmission cable. As shown in FIG. 1, a signal transmissioncable 1 comprises a first and second signal lines 3 a and 3 b, aninsulation layer 5, and a plating layer 7. The insulation layer 5 coversthe first and second signal lines 3 a and 3 b. As shown in FIG. 1, theinsulation layer 5 covers the first and second signal lines 3 a and 3 bcollectively. Each of the first and second signal lines 3 a and 3 b maybe made of strands, for example, and may be a twisted wire made bytwisting strands. The first and second signal lines 3 a and 3 b haveimproved flexibility if they are the twisted wires.

In the case where the signal transmission cable of the presentdisclosure is the differential signal transmission cable, it ispreferable that the maximum value of a differential-to-common modeconversion energy is −26 dB or less in a frequency band of 50 GHz andbelow. The differential-to-common mode conversion energy is measuredbefore winding the differential signal transmission cable around a drumor the like. In the signal transmission cable in the present disclosure,occurrence of a gap between the plating layer and the insulation layeris reduced. Therefore, the differential-to-common mode conversion energycan be reduced when the signal transmission cable in the presentdisclosure is provided as the differential signal transmission cable.

The signal transmission cable in the present disclosure may be used for,for example, signal transmission between electronic devices or signaltransmission between substrates in an electronic device. Examples of theelectronic devices comprise, for example, servers, routers and storagedevices handling high-speed signal transmission of several Gbps or more.The signal transmission cable in the present disclosure may be also usedas an acoustic cable, for example. The signal transmission cable in thepresent disclosure transmits a high-speed signal of, for example, 25 GHzand above.

(1-2) Insulation Layer

In a case where the signal transmission cable in the present disclosurecomprises two signal lines, it is preferable that the insulation layercovers the two signal lines collectively. To cover the two signal linescollectively means to cover both of the two signal lines together with asingle body of insulation layer. In a case where the two signal linesare covered collectively by one insulation layer, there will be no gapbetween separate insulation layers, as would be the case when eachsignal line is covered by a separte insulation layer. This helps reducevariation of dielectric constant along a longitudinal axis of the signaltransmission cable. Consequently, the differential-to-common modeconversion energy can be further reduced in the case where the signaltransmission cable in the present disclosure is provided as thedifferential signal transmission cable.

Further in the case where the insulation layer covers the two signallines collectively, the plating layer can be more uniformly formed on anouter peripheral surface of the insulation layer. Nevertheless, thefirst signal line and the second signal line of the two signal lines maybe covered individually by separate insulation layers.

Preferably, the shape of the outer periphery of the insulation layer maybe oval or elliptical on a section orthogonal to an axis of extension ofthe signal line. This helps form the plating layer uniformly over theentire outer peripheral surface of the insulation layer. Furthermore, itbecomes easier to perform surface roughening and surface improvementuniformly across the entire outer peripheral surface of the insulationlayer. The oval shape includes a shape that comprises two parallelstraight lines, and two arcs each connecting two ends of the straightlines.

The arithmetic average roughness Ra of the outer peripheral surface ofthe insulation layer is 0.6 μm or greater. This helps increase adhesionbetween the plating layer and the insulation layer, and also helpsreduce peeling of the plating layer from the insulation layer. Also, dueto the improved adhesion between the insulation layer and the platinglayer, occurrence of a gap between the insulation layer and the platinglayer is reduced. Consequently, a high shielding effect is exerted bythe plating layer. In the case where the signal transmission cable isthe differential signal transmission cable, the differential-to-commonmode conversion energy can be further reduced.

A method of providing the outer peripheral surface of the insulationlayer having the arithmetic average roughness Ra of 0.6 μm or greatermay include surface roughening treatment, such as blasting, immersion inan acidic or alkaline solution, immersion in a chromic acid solution,and immersion in a chelate solution.

Fine particles to be sprayed against an object of blasting includes, forexample, dry ice, metallic particles, carbon particles, oxide particles,carbide particles, and nitride particles. Dry ice particles arepreferred since they do not tend to remain in the insulation layer afterthe blasting.

In the blasting, the arithmetic average roughness Ra of the outerperipheral surface of the insulation layer increases as the speed forspraying the fine particle is increased. The arithmetic averageroughness Ra of the outer peripheral surface of the insulation layerincreases as the duration of the blasting lengthens. The arithmeticaverage roughness Ra of the outer peripheral surface of the insulationlayer increases as the distance between a tip of a nozzle to spray thefine particles and the outer peripheral surface of the insulation layeris shortened.

The arithmetic average roughness Ra of the outer peripheral surface ofthe insulation layer may preferably be 10 μm or less, or more preferably5 μm or less. A transmission loss can be reduced when the arithmeticaverage roughness Ra of the outer peripheral surface of the insulationlayer is 10 μm or less.

A method of measuring the arithmetic average roughness Ra may comprise,for example, a use of a laser microscope VK8500 by Keyence Corporation.An example of specific measuring conditions are explained below. Thatis, two positions opposite to each other on the outer peripheral surfaceof the insulation layer are chosen. The two positions are flat or havethe smallest curvature (hereinafter, the two positions are referred toas a first measuring position and a second measuring position). The“curvature” here may be a curvature of the outer peripheral surface ofthe insulation layer taken orthogonally to the longitudinal axis of thecable, for example. In addition, the “two positions having the smallestcurvature” here may be, for example, “two positions that provide theleast average curvature”. The curvature is the inverse number of theradius of curvature; thus the least curvature corresponds to thegreatest radius of curvature. A measuring area at the first measuringposition is determined. The measuring area has a shape of a rectanglehaving length of 150 μm along the longitudinal axis of the cable andlength of 120 μm along the circumference of the cable. The measuringarea in the shape of a rectangle at the first measuring position may be,for example, a rectangular measuring area with the first measuringposition situated at the center thereof or a rectangular measuring areathat comprises the first measuring position. A first arithmetic averageroughness Ra is measured in the determined measuring area by using theaforementioned laser microscope. A second arithmetic average roughnessRa is measured at the second measuring position likewise. Lastly, anaverage value of the first arithmetic average roughness Ra at the firstmeasuring position and the second arithmetic average roughness Ra at thesecond measuring position is calculated. The calculated average value isused as the arithmetic average roughness Ra of the outer peripheralsurface of the insulation layer. The arithmetic average roughness Ra isobtained before forming the plating layer.

Through the following test (hereinafter called a first test), it wasconfirmed that peeling of the plating layer from the insulation layercan be reduced when the arithmetic average roughness Ra of the outerperipheral surface of the insulation layer is 0.6 μm or greater.Firstly, a polyethylene (PE) substrate was prepared. This substratecorresponds to the insulation layer. Blasting using dry ice as the fineparticles (hereinafter called dry-ice-blasting) was applied to thesubstrate. The dry-ice-blasting corresponds to the surface rougheningtreatment. The arithmetic average roughness Ra of the surface of thesubstrate was 0.6 μm or greater after the dry-ice-blasting. Then, acorona discharge exposure process was performed on the substrate assurface modification treatment. Examples of the surface modificationtreatment may comprise electron beam irradiation, ion irradiation, thecorona discharge exposure, plasma exposure, ultraviolet irradiation,X-ray irradiation,y-ray irradiation, and immersion in ozone-containingliquid. An adhesion-wetting surface free energy of the surface of thesubstrate after the corona discharge exposure process was 66 mJ/m² orgreater and the contact angle was 95° or greater. A method of measuringthe adhesion-wetting surface free energy will be explained later.

After the corona discharge exposure process, a copper plating layer wasformed on the surface of the substrate by an electroless plating method.Then, cuts were made on the copper plating layer in a checkerboardpattern. The cuts penetrated through the copper plating layer andreached the substrate. An adhesive tape was attached to the copperplating layer and then removed. FIG. 2B shows the copper plating layerwhen the adhesive tape was removed. In FIG. 2B, reference numeral 181indicates the cuts. No peeling of the copper plating layer occurred inany section of the checkerboard pattern. This explains that the adhesionperformance between the copper plating layer and the substrate was high.

A comparative example was tested basically by a similar method. Nosurface roughening treatment or no surface modification treatment wereapplied to a substrate of the comparative example. The arithmeticaverage roughness Ra of a surface of the substrate was 0.13 μm. FIG. 2Ashows the copper plating layer of the comparative example when theadhesive tape was removed. The checkerboard pattern had 20 sections.Peeling of the copper plating layer was found in 17 sections, and thereoccurred portions 182 where the substrate was exposed. This explainsthat the adhesion performance between the copper plating layer and thesubstrate was low in the comparative example.

Preferably, the contact angle on the outer peripheral surface of theinsulation layer may be 95° or less. This helps easily make thethickness of the plating layer uniform. Uniform thickness of the platinglayer can reduce the transmission loss of the signal transmission cable.

A method of making the contact angle on the outer peripheral surface ofthe insulation layer at 95° or less may comprise the surfacemodification treatment, such as the electron beam irradiation, the ionirradiation, the corona discharge exposure, the plasma exposure, theultraviolet irradiation, the X-ray irradiation, the γ-ray irradiation,and the immersion in ozone-containing liquid.

In any of the aforementioned treatments, the contact angle decreases asthe intensity of treatment is increased, and the contact angle decreasesas the duration of treatment is lengthened. A method of increasing asurface modification effect by the corona discharge exposure maycomprise, for example, increase of voltage and increase of oxygenconcentration in the atmosphere of the corona discharge. A method ofmeasuring the contact angle is to drop a droplet of water having adiameter of 1.5 mm on the outer peripheral surface of the insulationlayer and read the contact angle. The contact angle is obtained beforeforming the plating layer.

Preferably, an absolute value of the adhesion-wetting surface freeenergy on the outer peripheral surface of the insulation layer may be 66mJ/m² or greater. This helps easily make the thickness of the platinglayer uniform. Uniform thickness of the plating layer reduces thetransmission loss of the signal transmission cable.

A method of providing the outer peripheral surface of the insulationlayer with the absolute value of the adhesion-wetting surface freeenergy thereon of 66 mJ/m² or greater may comprise the surfacemodification treatment, such as the electron beam irradiation, the ionirradiation, the corona discharge exposure, the plasma exposure, theultraviolet irradiation, the X-ray irradiation, the y-ray irradiation,and the immersion in ozone-containing liquid.

In any of the aforementioned treatments, the absolute value of theadhesion-wetting surface free energy can be increased as the intensityof treatment is increased, and the absolute value of theadhesion-wetting surface free energy can be increased as the duration oftreatment is lengthened.

An absolute value of an adhesion-wetting surface free energy AG Δscalculated by the following Formula 3.

|ΔG|=|−γ_(LG)(cos θ+1)|  [Formula 3]

The γ_(LG) in Formula 3 is a constant value, which is 72.75 mJ/m². Thevalue θ is the contact angle on the outer peripheral surface of theinsulation layer. The adhesion-wetting surface free energy ΔG isobtained before forming the plating layer.

Through the following test, it was confirmed that the plating layer canbe uniformly formed when the contact angle on the outer peripheralsurface of the insulation layer is 95° or less or when the absolutevalue of the adhesion-wetting surface free energy is 66 mJ/m² orgreater.

Firstly, a polyethylene substrate was prepared. This substratecorresponds to the insulation layer. The dry-ice-blasting was applied tothe substrate, and then the corona discharge exposure process wasapplied to the substrate. The dry-ice-blasting corresponds to thesurface roughening treatment. The corona discharge exposure processcorresponds to the surface modification treatment. After the coronadischarge exposure process, the arithmetic average roughness Ra of thesurface of the substrate was 0.6 μm or greater, the absolute value ofthe adhesion-wetting surface free energy on the surface of the substratewas 66 mJ/m² or greater, and the contact angle was 95° or less. Then,the copper plating layer was formed on the surface of the substrate byan electroplating method. The thickness of the copper plating layer wasset to be three times thicker than the copper plating layer formed inthe first test. FIG. 3C shows the formed copper plating layer. Thecopper plating layer was formed uniformly. The adhesion performancebetween the copper plating layer and the substrate was high so that nopeeling of the copper plating layer occurred.

A comparative example was prepared basically by a similar method. In thecomparative example, the arithmetic average roughness Ra of the surfaceof the substrate was less than 0.6 μm after the surface rougheningtreatment. After the surface modification treatment, the absolute valueof the adhesion-wetting surface free energy on the surface of thesubstrate was 66 mJ/m² or greater, and the contact angle was 95° orless. FIG. 3A shows the copper plating layer formed in the comparativeexample. Significant peeling of the copper plating layer occurred.

A reference example was also prepared basically by a similar method. Inthe reference example, the arithmetic average roughness Ra of thesurface of the substrate was 0.6 μm or greater after the surfaceroughening treatment. After the surface modification treatment, theabsolute value of the adhesion-wetting surface free energy on thesurface of the substrate was less than 66 mJ/m², and the contact anglewas greater than 95°. FIG. 3B shows the copper plating layer formed inthe reference example. No peeling of the copper plating layer occurred.However, a plating failure bulge 191 called a blister existed in thesurface of the copper plating layer and the plated surface was notuniform.

With the process of first applying the dry-ice-blasting to the outerperipheral surface of the insulation layer and then conducting thesurface modification treatment by the corona discharge exposure process(hereinafter called a specific surface modification treatment), thearithmetic average roughness Ra of the outer peripheral surface of theinsulation layer and the contact angle or the absolute value of theadhesion-wetting surface free energy can be controlled. This wasconfirmed by the following test. The dry-ice-blasting corresponds to thesurface roughening treatment. The corona discharge exposure processcorresponds to the surface modification treatment.

The specific surface modification treatment was applied to apolyethylene substrate. The substrate corresponds to the insulationlayer. There are several conditions for the specific surfacemodification treatment. FIG. 4 shows a correlation between thearithmetic average roughness Ra and the contact angle after the specificsurface modification treatment is applied. The arithmetic averageroughness Ra of 0.6 μm or greater and the contact angle of 95° or lesswere achieved by conducting the specific surface modification treatment.

FIG. 5 shows a correlation between the arithmetic average roughness Raand the adhesion-wetting surface free energy after the specific surfacemodification treatment is applied. The arithmetic average roughness Raof 0.6 μm or greater and the absolute value of the adhesion-wettingsurface free energy of 66 mJ/m² or greater were achieved by conductingthe specific surface modification treatment.

Preferably, the outer peripheral surface of the insulation layer mayhave concavities. This helps reduce occurrence of peeling of the platinglayer from the insulation layer. The concavity preferably may have, atits bottom in the depth direction, a space that is wider than an openingof the concavity. In this case, following effects can be exerted. Thatis, a plating liquid reacheds the bottom of the concavity when forming aplating layer in a plating bath. This then generates nucleus at thebottom of the concavity, causing growth of the plating layer also at thebottom of the concavity. The plating layer grown at the bottom of theconcavity is wider than the opening of the concavity and thus unlikelyto come out from the opening. Thus, an anchoring effect is exerted, andpeeling of the plating layer from the insulation layer becomes lesslikely to happen.

A method of forming concavities in the insulation layer may compriseblasting. For example, the dry-ice-blasting can be used as the blasting.The dry-ice-blasting corresponds to the surface roughening treatment.FIG. 6 shows an example of a concavity formed in the outer peripheralsurface of the insulation layer by the dry-ice-blasting. FIG. 6 is asectional view taken near an outer peripheral surface 72 of aninsulation layer 71. A concavity 73 is formed in the outer peripheralsurface 72. The concavity 73 comprises a space at its bottome in thedepth direction that is wider than an opening 74 of the concavity 73.The shape of the concavity 73 is similar to an octopus pot. In theexample shown in FIG. 6, the insulation layer 71 is made ofpolyethylene.

The material of the insulation layer may be selected from, for example,polytetrafluoroethylene (PTFE), perfluoroalkoxy (PFA),perfluoroethylene-propene copolymer (FEP), ethylene tetrafluoroethylenecopolymer (ETFE), tetrafluoroethylene-perfluorodioxole copolymer(TFE/PDD), polyvinylidene fluoride (PVDF), polychlorotrifluoroethylene(PCTFE), ethylene-chlorotrifluoroethylene copolymer (ECTFE), polyvinylfluoride (PVF), silicone, and polyethylene (PE).

The material of the insulation layer may be a foamable resin. In thiscase, the dielectric constant and a dielectric loss tangent of theinsulation layer decrease. A method of manufacturing the insulationlayer from the foamable resin may comprise, for example, mixing andkneading the resin with a foaming agent and foaming the kneaded materialby controlling temperature and pressure when molding the insulationlayer. Another manufacturing method of the insulation layer from thefoamable resin may comprise, for example, injecting nitrogen gas orother gas into the resin when high-pressure molding the insulation layerand then reducing the pressure to create foams.

The insulation layer may also be manufactured from the foamable resin asdescribed below. An extrusion die having a desired shape is placed in anextruder, and the signal line is extruded together with the foamableresin from the extruder, thereby forming the insulation layer with thefoamable resin.

Preferably, the insulation layer may be made of polyethylene forexample, and a crystallinity X_(c) defined by the following Formulal 1may be 0.744 or greater. This makes it easy to provide uniform thicknessof the plating layer. Uniform thickness of the plating layer helpsreduce the transmission loss of the signal transmission cable.

$\begin{matrix}{X_{c} = \frac{I_{c}}{I_{c} + I_{a}}} & \left\lbrack {{Formula}\mspace{14mu} 1} \right\rbrack\end{matrix}$

I_(c) in Formula 1 is X-ray diffraction intensity of a crystallinecomponent, and I_(a) is X-ray diffraction intensity of a noncrystallinecomponent.

A method of making the crystallinity X_(c) of the polyethyleneinsulation layer be 0.744 or greater may comprise the surfacemodification treatment, such as the electron beam irradiation, the ionirradiation, the corona discharge exposure, the plasma exposure, theultraviolet irradiation, the X-ray irradiation, the γ-ray irradiation,and the immersion in ozone-containing liquid. In any of theaforementioned treatment, the crystallinity X_(c) can be increased asthe intensity of treatment is increased, and the crystallinity X_(c) canbe increased as the duration of treatment is lengthened.

I_(c) and I_(a) in Formula 1 are calculated as follows. An X-raydiffraction pattern of an insulation layer specimen is obtained byusing, for example, an X-ray diffraction apparatus RINT2500 by RigakuCorporation. FIG. 7 and FIG. 8 show example X-ray diffraction patterns.In the X-ray diffraction patterns shown in FIG. 7 and FIG. 8, thehorizontal axes are diffraction angle 2θ. The range of the diffractionangle 2θ in these X-ray diffraction patterns is between 13° and 21°inclusive.

In these X-ray diffraction patterns, a broad halo (hereinafter called anoncrystalline halo) that has a diffraction peak near 16.4° to 16.5°corresponds to the noncrystalline component. Sharp spectra (hereinaftercalled crystalline component spectra) that have a diffraction peak at17.7° corresponds to the crystalline component.

Spectral fitting using the Lorentzian function is applied to thenoncrystalline halo to obtain a smooth curve F_(a) that satisfactorilymatches the noncrystalline halo. The obtained curve F_(a) is shown inFIG. 7 and FIG. 8. The intensity of the noncrystalline halo obtained bycalculation of integrated intensity based on the curve F_(a), is denotedwith I_(a).

Spectral fitting using the Lorentzian functions is applied to thecrystalline component spectra to obtain a smooth curve F_(c) thatsatisfactorily matches the crystalline component spectra. The obtainedcurve F_(c) is shown in FIG. 7 and FIG. 8. The intensity ofnoncrystalline component spectra obtained by the calculation ofintegrated intensity based on this curve F_(c), is denoted with I_(c).The crystallinity X_(c) is obtained before forming the plating layer.

Preferably, the insulation layer may be made of FEP for example, and thecrystallinity X_(c) of the insulation layer expressed in the followingFormula 1 may be 0.47 or less. This helps provide uniform thickness ofthe plating layer. Uniform thickness of the plating layer helps reducethe transmission loss of the signal transmission cable.

$\begin{matrix}{X_{c} = \frac{I_{c}}{I_{c} + I_{a}}} & \left\lbrack {{Formula}\mspace{14mu} 1} \right\rbrack\end{matrix}$

I_(c) in Formula 1 is the X-ray diffraction intensity of the crystallinecomponent, and I_(a) is the X-ray diffraction intensity of thenoncrystalline component. I_(c) and I_(a) are calculated by theaforementioned method. The crystallinity X_(c) is obtained beforeforming the plating layer.

A method of making the crystallinity X_(c) of the FEP insulation layerbe 0.47 or less may comprise the surface modification treatment, such asthe electron beam irradiation, the ion irradiation, the corona dischargeexposure, the plasma exposure, the ultraviolet irradiation, the X-rayirradiation, the γ-ray irradiation, and the immersion inozone-containing liquid. In any of the aforementioned treatment, thecrystallinity X_(c) can be increased as the intensity of treatment isincreased, and the crystallinity X_(c) can be increased as the durationof treatment is lengthened.

Through the following test, it was confirmed that there is a correlationbetween the crystallinity X_(c) and the contact angle. The specificsurface modification treatment was applied to the polyethylenesubstrate. The substrate corresponds to the insulation layer. There areseveral conditions for the specific surface modification treatment. FIG.9A shows the correlation between the crystallinity X_(c) and the contactangle after the specific surface modification treatment is applied. Thecontact angle became significantly small when the crystallinity X_(c)was 0.744 or greater.

The specific surface modification treatment was applied to the FEPsubstrate. The substrate corresponds to the insulation layer. There areseveral conditions for the specific surface modification treatment. FIG.9B shows the correlation between the crystallinity X_(c) and the contactangle after the specific surface modification treatment is applied. Thecontact angle became significantly small when the crystallinity X_(c)was 0.47 or less.

Preferably, the insulation layer may comprise the followingconfiguration, for example. The insulation layer comprises polyethylene.The polyethylene has crystal structures of the triclinic crystal systemor of the orthorhombic system, or of a coexisting state of at least oneof these crystal structures. The polyethylene has preferentialcrystalline orientations in specific two or less number of crystal axes.A (100) crystalline orientation degree O₁₀₀ of the polyethyleneexpressed in the following Formula 2 is 0.26 or less. This insulationlayer is hereinafter called a polyethylene insulation layer withspecific orientation.

$\begin{matrix}{O_{100} = \frac{I_{200}}{I_{110} + I_{200}}} & \left\lbrack {{Formula}\mspace{14mu} 2} \right\rbrack\end{matrix}$

I₂₀₀ in Formula 2 is the X-ray diffraction intensity of the index 200,and I₁₁₀ is the X-ray diffraction intensity of the index 110.

The thickness of the plating layer can be made uniform easily when theinsulation layer is the polyethylene insulation layer with specificorientation. Uniform thickness of the plating layer helps reduce thetransmission loss of the signal transmission cable.

A method of obtaining the polyethylene insulation layer with specificorientation as the insulation layer may comprise the surfacemodification treatment, such as the electron beam irradiation, the ionirradiation, the corona discharge exposure, the plasma exposure, theultraviolet irradiation, the X-ray irradiation, the γ-ray irradiation,and the immersion in ozone-containing liquid.

In any of the aforementioned treatments, the crystalline orientationdegree O₁₀₀ can be decreased as the intensity of treatment is increased,and the crystalline orientation degree O₁₀₀ can be decreased as theduration of treatment is lengthened.

I₂₀₀ and I₁₁₀ in Formula 2 are calculated as follows.

An X-ray diffraction pattern of an insulation layer specimen is obtainedby using, for example, an X-ray diffraction apparatus RINT2500 by RigakuCorporation. FIG. 10 and FIG. 11 show example X-ray diffractionpatterns. In the X-ray diffraction patterns shown in FIG. 10 and FIG.11, horizontal axes represent the diffraction angle 2θ. The range of thediffraction angle 2θ in the X-ray diffraction patterns is between 19°and 26° inclusive. FIG. 10 and FIG. 11 show the X-ray diffractionpatterns of the polyethylene insulation layer specimens having acrystalline structure of the orthorhombic system. FIG. 10 shows an X-raydiffraction pattern of polyethylene with no surface modificationtreatment. FIG. 11 shows an X-ray diffraction pattern of polyethylenehaving undergone the corona discharge exposure process as the surfacemodification treatment.

Diffraction spectra having a peak around 21.5° (hereinafter called 110diffraction spectra) correspond to the Miller index 110, which means thenotation of crystallography for 110 plane in crystal lattice.Diffraction spectra having a peak around 23.8° (hereinafter called 200diffraction spectra) corresponds to the Miller index 200, which meansthe notation of crystallography for 200 plane in crystal lattice.

Spectral fitting using the Lorentzian function is applied to the 110diffraction spectra to obtain a smooth curve F₁ that satisfactorilymatches the 110 diffraction spectra. The obtained curve F₁ is shown inFIG. 10 and FIG. 11. The intensity of the 110 diffraction spectra,obtained by the calculation of integrated intensity based on the curveF₁, will be denoted with I₁₁₀ hereafter.

Spectral fitting using the Lorentzian function is applied to the 200diffraction spectra to obtain a smooth curve F₂ that satisfactorilymatches the 200 diffraction spectra. The obtained curve F₂ is shown inFIG. 10 and FIG. 11. The intensity of the 200 diffraction spectra,obtained by the calculation of integrated intensity based on the curveF₂, will be denoted with I₂₀₀ hereafter.

If each crystal particle comprised in a material including apolycrystalline substance is preferentially oriented in a particulardirection, then the X-ray diffraction intensity on a particular indexplane is relatively high compared to the X-ray diffraction intensity onanother index plane. Accordingly, orientation of a specific latticeplane can be quantified by a ratio of the X-ray diffraction intensity.The (100) crystal orientation degree O₁₀₀ is the ratio of the X-raydiffraction intensity and also shows preferential orientation of a (100)plane.

Through the following test, it was confirmed that there is a correlationbetween the (100) crystal orientation degree O₁₀₀ and the contact angle.The specific surface modification treatment was conducted on thepolyethylene substrate. The substrate corresponds to the insulationlayer. There are several conditions for the specific surfacemodification treatment. FIG. 12 shows the correlation between the (100)crystal orientation degree O₁₀₀ and the contact angle after the specificsurface modification treatment is applied. The contact angle becamesignificantly small when the (100) crystal orientation degree O₁₀₀ is0.26 or less. The (100) crystal orientation degree O₁₀₀ is obtainedbefore forming the plating layer.

Preferably, the insulation layer may comprise the followingconfiguration, for example. The insulation layer is made ofpolyethylene. The polyethylene has a crystalline size of 18 nm orgreater in the crystalline component. If the insulation layer isconfigured as mentioned above, the thickness of the plating layer can beeasily made uniform. Uniform thickness of the plating layer can helpreduce the transmission loss of the signal transmission cable.

A method of obtaining the aforementioned insulation layer may comprisethe surface modification treatment, such as the electron beamirradiation, the ion irradiation, the corona discharge exposure, theplasma exposure, the ultraviolet irradiation, the X-ray irradiation, theγ-ray irradiation, and the immersion in ozone-containing liquid.

In any of the aforementioned treatments, the crystalline size of thepolyethylene in the crystalline component can be increased as theintensity of treatment is increased, and the crystalline size of thepolyethylene in the crystalline component can be increased as theduration of treatment is lengthened.

The crystalline size of the polyethylene in the crystalline component isexpressed by the following Formula 4.

$\begin{matrix}{D = \frac{K\; \lambda}{B\; \cos \; \theta}} & \left\lbrack {{Formula}\mspace{14mu} 4} \right\rbrack\end{matrix}$

D in Formula 4 is the crystalline size of the polyethylene in thecrystalline component. K is the Scherrer constant. The value of K is setto 2/π. λ is the X-ray wavelength. B is the spreading width of the X-raydiffraction peak. θ is the X-ray diffraction angle. λ, B, and θ arevalues that can be obtained from the X-ray diffraction pattern of aninsulation layer specimen. The crystalline size is obtained beforeforming the plating layer.

Through the following test, it was confirmed that there is a correlationbetween the crystalline size D of the polyethylene in the crystallinecomponent and the contact angle. The specific surface modificationtreatment was conducted on the polyethylene substrate. The substratecorresponds to the insulation layer. There are several conditions forthe specific surface modification treatment. FIG. 13A shows thecorrelation between the crystalline size D in the crystalline componentof the polyethylene and the contact angle after the specific surfacemodification treatment is applied. The contact angle becamesignificantly small when the crystalline size D in the crystallinecomponent of the polyethylene was 18 nm or greater.

Preferably, the insulation layer may comprise the followingconfiguration, for example. The insulation layer is made of FEP. Thecrystalline size in the crystalline component of FEP is 13.6 nm or less.If the insulation layer is configured as mentioned above, the thicknessof the plating layer can be easily made uniform. Uniform thickness ofthe plating layer can help reduce the transmission loss of the signaltransmission cable.

A method of obtaining the aforementioned insulation layer may comprisethe surface modification treatment, such as the electron beamirradiation, the ion irradiation, the corona discharge exposure, theplasma exposure, the ultraviolet irradiation, the X-ray irradiation, they-ray irradiation, and the immersion in ozone-containing liquid.

In any of the aforementioned treatments, the crystalline size in thecrystalline component of FEP can be increased as the intensity oftreatment is increased, and the crystalline size in the crystallinecomponent of FEP can be increased as the duration of treatment islengthened. A method of calculating the crystalline size in thecrystalline component of FEP is the same as the method of calculatingthe crystalline size in the crystalline component of the polyethylene.The crystalline size is obtained before forming the plating layer.

There is a correlation between the crystalline size D in the crystallinecomponent of FEP and the contact angle. This was confirmed by thefollowing test. The specific surface modification treatment wasconducted on the FEP substrate. The substrate corresponds to theinsulation layer. There are several conditions for the specific surfacemodification treatment. FIG. 13B shows the correlation between thecrystalline size D in the crystalline component of FEP and the contactangle after the specific surface modification treatment is applied. Thecontact angle became significantly small when the crystalline size D inthe crystalline component of FEP was 13.6 nm or less.

(1-3) Plating Layer

Preferably, the thickness of the plating layer may be between 1 μm and 5μm inclusive. The shielding effect by the plating layer is significantlyhigh when the thickness of the plating layer is 1 μm or greater. Withsuch a thickness, an intra-pair skew can be decreased further and thedifferential-to-common mode conversion energy can be reduced further inthe case where the signal transmission cable in the present disclosureis the differential signal transmission cable. Thedifferential-to-common mode conversion energy can be particularlysignificantly reduced in a case of transmitting a signal of 25 GHz orhigher.

Time required to form the plating layer can be shortened when thethickness of the plating layer is 5 μm or less. Flexibility of thesignal transmission cable can be improved when the thickness of theplating layer is 5 μm or less. An outer diameter of the signaltransmission cable can be small when the thickness of the plating layeris 5 μm or less. The thickness of the plating layer can be controlled bya publicly known method. For example, the plating layer can be madethicker as the time for electroplating and/or electroless-plating islengthened. The plating layer can be made thicker as the current for theelectroplating is increased.

Preferably, a standard deviation of the thickness of the plating layermay be 0.8 μm or less. This can reduce the transmission loss of thesignal transmission cable, and also reduce noise further sinceextraordinally thin portion of the plating layer is hardly produced.

The standard deviation of the thickness of the plating layer may becalculated by the following method. Four cross-sections are taken atfour positions on the differential signal transmission cable. Eachcross-section is taken orthogonally to the longitudinal direction of thedifferential signal transmission cable. The distance between theadjacent cross-sections is 3 m. Four points are randomly selected oneach of the four cross-sections, providing sixteen points in total formeasuring the thickness of the plating layer. A standard deviation ofall the measured thicknesses at the sixteen points is used as thestandard deviation of the thickness of the plating layer.

For example, the standard deviation of the thickness of the platinglayer can be reduced by decreasing the contact angle on the outerperipheral surface of the insulation layer or by increasing the absolutevalue of the adhesion-wetting surface free energy.

The contact angle on the outer peripheral surface of the insulationlayer can be decreased and the absolute value of the adhesion-wettingsurface free energy can be increased by conducting the surfacemodification treatment on the insulation layer. The surface modificationtreatment may be, for example, the electron beam irradiation, the ionirradiation, the corona discharge exposure, the plasma exposure, theultraviolet irradiation, the X-ray irradiation, the y-ray irradiation,and the immersion in ozone-containing liquid.

The plating layer may comprise stacked layers. The number of the stackedlayers may be 2, 3, or 4 or more. A part of the stacked layers may be amagnetic layer comprising ferrite, for example, and another part of thestacked layers may be a nonmagnetic layer comprising copper, forexample. This enables the plating layer to exert the shielding effectagainst ferromagnetic fields and weak magnetic fields. The plating layercan exert the shielding effect against noise in a low frequency band ina range from several tens of MHz to several hundreds of MHz, and alsoagainst noise in a high frequency band of several tens of GHz.

The plating layer may be formed, for example, through theelectroless-plating process followed by the electroplating process. Theplating layer can be easily formed on the insulation layer by thismethod. The method also requires less time to form the plating layerthan a method of forming the entire plating layer by theelectroless-plating process.

2. Method of Manufacturing Signal Transmission Cable

The signal transmission cable in the present disclosure can bemanufactured by the following method, for example. FIG. 14 shows amanufacturing system 101 used to manufacture the differential signaltransmission cable. The differential signal transmission cablecorresponds to the signal transmission cable. The manufacturing system101 comprises a degreasing unit 103, a wet etching unit 105, a firstactivation unit 107, a second activation unit 109, anelectroless-plating unit 111, an electroplating unit 113, and a conveyerunit 115.

The degreasing unit 103 comprises a degreasing bath 117, and a degreaser119. The degreaser 119 is contained in the degreasing bath 117. Thedegreaser 119 comprises at least one of sodium borate, sodium phosphateor surfactant, for example. The temperature of the degreaser 119 is in arange, for example, from 40° C. to 60° C.

The wet etching unit 105 for the surface roughening treatment comprisesan etching bath 121 and an etching solution 123. The etching solution123 is contained in the etching bath 121. The etching solution 123comprises at least one of chromic acid, sulfuric acid, ozone, acid,alkali or chelate, for example. The temperature of the etching solution123 is in a range, for example, from 65° C. to 70° C.

The first activation unit 107 comprises a first activation bath 125 anda first liquid activator 127. The first liquid activator 127 iscontained in the first activation bath 125. The first liquid activator127 comprises at least one of palladium chloride, tin (II) chloride, orconcentrated hydrochloric acid, for example. The temperature of thefirst liquid activator 127 is in a range, for example, from 30° C. to40° C.

The second activation unit 109 comprises a second activation bath 129and a second liquid activator 131. The second liquid activator 131 iscontained in the second activation bath 129. The second liquid activator131 comprises sulfuric acid, for example. The temperature of the secondliquid activator 131 is in a range, for example, from 0° C. to 50° C.

The electroless-plating unit 111 comprises an electroless-plating bath133 and an electroless-plating liquid 135. The electroless-platingliquid 135 is contained in the electroless-plating bath 133. Theelectroless-plating liquid 135 comprises, for example, copper sulfate,Rochelle salt, formaldehyde, and sodium hydroxide. The temperature ofthe electroless-plating liquid 135 is in a range, for example, from 20°C. to 30° C.

The electroplating unit 113 comprises an electroplating bath 137, anelectroplating liquid 139, anodes 141, and a power source unit 143. Theanodes 141 comprise a first anode 141 a and a second anode 141 b. Theelectroplating liquid 139 is contained in the electroplating bath 137.The electroplating liquid 139 comprises a composition as shown in Table1 or Table 2, for example. The temperature of the electroplating liquid139 is in a range, for example, from 20° C. to 25° C.

TABLE 1 Composition of Copper Sulfate Plating Bath Composition ofChemical Concentration Plating Bath Formula (g/l) Copper sulfateCuSO₄•5H₂O  60~250 Metallic copper Cu 15~70 Sulfuric acid H₂SO₄  25~220Chloride ion Cl⁻ 0.02~0.2  (Sodium chloride, (NaCl, HCl) Hydrochloricacid*)

TABLE 2 Composition of Copper Cyanide Plating Bath Composition ofChemical Concentration Plating Bath Formula (g/l) Copper(I) cyanide CuCN20~80 Sodium cyanide NaCN  25~130 (Potassium cyanide) (KCN) Free sodiumcyanide* NaCN  5~25 (Free potassium cyanide) (KCN) Pottasium sodiumKNaC₄H₄O₆•4H₂O 15~60 tartrate Sodium carbonate Na₂CO₃ 10~30 (Pottasiumcarbonate) (K₂CO₃) Pottasium hydroxide KOH 10~20 (Sodium hydroxide)(NaOH)

The anodes 141 are immersed in the electroplating liquid 139. The anodes141 are produced by, for example, casting and roll-forging of moltencopper produced from copper melt. Alternatively, the anodes 141 may beproduced by the following method. Starting-sheet electrolysis isperformed using a crude copper as an anode and a stainless-steel ortitanium as a cathode. Pure copper plates deposited on the surface ofthe cathode are removed and used as the anodes 141. The power sourceunit 143 applies a direct current voltage between the anodes 141 andbobbins 165 and 169, which will be explained later.

The conveyer unit 115 comprises bobbins 145, 147, 149, 151, 153, 155,157, 159, 161, 163, 165, 167, and 169. Hereinafter, these bobbins mayalso be referred to as bobbins collectively. The bobbins 165 and 169 areelectrically conductive. The bobbin 167 has an insulating property.

As shown in FIG. 14, the bobbins are basically arranged in series alonga conveying direction CD. The conveying direction CD is the directionfrom the degreasing unit 103 toward the electroplating unit 113 via thewet etching unit 105, the first activation unit 107, the secondactivation unit 109, and the electroless-plating unit 111 in this order.

A part of the bobbin 147 is immersed in the degreaser 119. A part of thebobbin 151 is immersed in the etching solution 123. Apart of the bobbin155 is immersed in the first liquid activator 127. A part of the bobbin159 is immersed in the second liquid activator 131. A part of the bobbin163 is immersed in the electroless-plating liquid 135. The bobbin 167 isentirely immersed in the electroplating liquid 139.

The conveyer unit 115 continuously conveys the differential signaltransmission cable 171 through the bobbins in the conveying directionCD. The differential signal transmission cable 171 has the signalline(s) and the insulation layer in an initial state, but the platinglayer is not yet formed. The insulation layer may be prepared by, forexample, a publicly known extrusion molding.

In the conveyance, the differential signal transmission cable 171 isfirst immersed in the degreaser 119 in the degreasing unit 103 for 3 to5 minutes. Thus, grease smeared on the surface of the insulation layeris removed.

The differential signal transmission cable 171 is then immersed in theetching solution 123 in the wet etching unit 105 for 8 to 15 minutes.Thus, irregularities are formed on the outer peripheral surface of theinsulation layer. Also, functional groups, such as a carbonyl group anda hydroxy group, are formed on the outer peripheral surface of theinsulation layer. Consequently, the outer peripheral surface of theinsulation layer is hydrophilized, which improves the surfacewettability.

The differential signal transmission cable 171 is then immersed in thefirst liquid activator 127 in the first activation unit 107 for 1 to 3minutes. Thus, a catalytic layer is formed on the outer peripheralsurface of the insulation layer.

The differential signal transmission cable 171 is then immersed in thesecond liquid activator 131 in the second activation unit 109 for 3 to 6minutes. Thus, a surface of the catalytic layer is cleaned.

The differential signal transmission cable 171 is then immersed in theelectroless-plating liquid 135 in the electroless-plating unit 111. Animmersion time is, for example, 10 minutes or shorter. Anelectroless-plating layer is thus formed on the outer peripheral surfaceof the insulation layer. The electroless-plating layer corresponds tothe plating layer. The electroless-plating layer becomes thicker as theimmersion time in the electroless-plating liquid 135 lengthens.

The differential signal transmission cable 171 is then immersed in theelectroplating liquid 139 in the electroplating unit 113. An immersiontime is, for example, 3 minutes or shorter. An electroplating layer isthus formed on the outer peripheral surface of the electroless-platinglayer. The electroplating layer corresponds to the plating layer. Theelectroplating layer becomes thicker as the immersion time in theelectroplating liquid 139 lengthens. Table 3 shows specific conditionsfor the electroplating in the electroplating unit 113.

Manufacture of the differential signal transmission cable 171 iscompleted through the aforementioned steps.

TABLE 3 Conditions for Electroplating by Copper Sulfate Plating BathItems Control Value Bath temperature (° C.) 20~25 Cathode currentdensity 1~6 (A/dm²) Anode current density ~2.5 (A/dm²) Bath voltage (V)1~6 Agitation method Air agitation Filteration Continuous filteration,at least 3 times/hour Anode Phosphorized copper Anode bag Saran ® fiber,etc.

Although it is omitted in FIG. 14, the differential signal transmissioncable 171 is cleaned in pure water between the units. A method ofcleaning may include ultrasonic cleaning, oscillation cleaning, andrunning water cleaning. The cleaning with pure water helps reduceresidues of agents used in a previous unit being brought into asubsequent unit.

Conveying speed of the differential signal transmission cable 171 may beappropriately adjusted. The conveying speed may be changed during theconveyance, or the conveyance may be suspended.

The differential signal transmission cable may be manufactured by usinga manufacturing system 201 shown in FIG. 15. The structure of themanufacturing system 201 is basically the same as the manufacturingsystem 101, but is partially different from the manufactruing system101. In the explanations hereinafter, such differences will be focused.The manufacturing system 201 comprises a surface improving unit 203 butno degreasing unit 103 or wet etching unit 105. FIG. 16 shows a detailedconfiguration of the surface improving unit 203.

The surface improving unit 203 comprises a housing 204, a fine-shapeforming device 205, and a hydrophilic treatment device 207. The housing204 houses components of the surface improving unit 203. Along theconveying direction CD, the housing 204 comprises an inlet 204A in itsupstream end, and an outlet 204B in its downstream end.

The conveyer unit 115 comprises, in the housing 204, four bobbins 209,211, 213, and 215. The bobbin 145 guides the differential signaltransmission cable 171 into the housing 204 through the inlet 204A. Thedifferential signal transmission cable 171, guided into the housing 204,is then conveyed through a figure-eight path from the bobbin 209 to thebobbin 211 and returns to the bobbin 209. The differential signaltransmission cable 171 is then conveyed from the bobbin 209 to thebobbin 213, then conveyed through another figure-eight path from thebobbin 213 to the bobbin 215, and returns to the bobbin 213. Thedifferential signal transmission cable 171 is then guided to exit fromthe outlet 204B to the bobbin 153 and conveyed to the first activationunit 107.

The fine-shape forming device 205 blasts fine particles of dry ice froma nozzle 205A against the differential signal transmission cable 171that is situated between the bobbin 209 and the bobbin 211. The blast isdriven by an air pressure. The arithmetic average roughness Ra of theouter peripheral surface of the insulation layer increases due to thecollision with the fine particles of dry ice. The fine-shape formingdevice 205 performs the dry-ice-blasting accordingly. Thedry-ice-blasting corresponds to the surface roughening treatment.

The outer peripheral surface of the insulation layer turns its firstside to the nozzle 205A when conveyed from the bobbin 209 to the bobbin211 and turns its second side to the nozzle 205A when returned from thebobbin 211 to the bobbin 209. Accordingly, the fine-shape forming device205 can help increase the arithmetic average roughness Ra across theentire area of the outer peripheral surface of the insulation layer.

Particle diameters of the fine particles of dry ice, a distance betweenthe tip of the nozzle 205A and the differential signal transmissioncable 171, and other particulars can be appropriately determined. Atemperature of the differential signal transmission cable 171 is, forexample, 20° C.

Conditions of the dry-ice-blasting may be appropriately changed. Theconditions may comprise, for example, the particle diameters of the fineparticles of dry ice, a flow amount of the dry ice, the air pressure,the distance between the tip of the nozzle 205A and the differentialsignal transmission cable 171, the conveying speed of the differentialsignal transmission cable 171, and the temperature of the differentialsignal transmission cable 171. For example, the dry-ice-blasting may beconducted at a temperature lower than a glass transition temperature ofa material of the insulation layer. Such a temperature may be, forexample, between −79° C. and 20° C. inclusive. The position of thenozzle 205A may be fixed, may oscillate, or may scan.

The hydrophilic treatment device 207 performs hydrophilic treatment bythe corona discharge exposure. The corona discharge exposure processcorresponds to the surface modification treatment. As shown in FIG. 16,the hydrophilic treatment device 207 comprises four plate electrodes 208(first to fourth plate electrodes 208 a, 208 b, 208 c, and 208 d). Thefirst and second plate electrodes 208 a and 208 b face each other acrossthe differential signal transmission cable 171 that is conveyed from thebobbin 213 to the bobbin 215. The third and fourth plate electrodes 208c and 208 d face each other across the differential signal transmissioncable 171 that returns from the bobbin 215 to the bobbin 213. Coronadischarge is generated by applying a high-frequency high voltage betweenthe facing plate electrodes 208. Exposure to the corona dischargehydrophilizes the outer peripheral surface of the insulation layer andimproves the wettability. Improved wettability decreases the contactangle and increases the absolute value of the adhesion-wetting surfacefree energy.

A mechanism that the corona discharge exposure hydrophilizes the outerperipheral surface of the insulation layer and improves the wettabilityis assumed as below. High energy electrons generated by the coronadischarge exposure ionize and/or dissociate oxygen molecules in the airand produce oxygen radical, ozone, and so forth. Simultaneously, thehigh energy electrons that reach close proximity of the outer peripheralsurface of the insulation layer cut and cleave main chains and sidechains of, for example, polyethylene and FEP comprised in the insulationlayer. The aforementioned oxygen radical, ozone, and so forth generatedby the corona discharge are recombined with these cleaved main chainsand side chains to form polar functional groups, such as the hydroxygroup and the carbonyl group, on the outer peripheral surface of theinsulation layer. As a result, the outer peripheral surface of theinsulation layer is hydrophilized and its wettability is improved.

For example, the voltage applied in the corona discharge exposureprocess is in a range from 2 to 14 kV, and the frequency is 15 kHz. Thedistance between the outer peripheral surface of the insulation layerand each of the plate electrodes 208 is, for example, between 0.1 mm and3 mm inclusive. The ambience inside the housing 204 is, for example, theatmospheric air.

The conditions of the corona discharge exposure may be appropriatelychanged. The conditions may comprise, for example, a magnitude of theapplied voltage, a frequency of the applied voltage, a distance betweenthe outer peripheral surface of the insulation layer and each of theplate electrodes 208, and the ambience inside the housing 204. Theambience inside the housing 204 may include oxygen, nitrogen, carbondioxide, and a rare gas. A material such as a silicone rubber may beinterposed between the outer peripheral surface of the insulation layerand each of the plate electrodes 208. In this case, each of the plateelectrodes 208 contacts the insulation layer indirectly and slidesagainst the silicone rubber during the corona discharge.

In addition, an exhaust ventilation system to discharge the air insidethe housing 204 and/or a dryer to dry an inside space of the housing 204may be arranged. This helps reduce rust of the differential signaltransmission cable 171. A neutralization apparatus may also be arrangedin the housing 204. This helps reduce static electricity in the housing204.

As mentioned above, the method of manufacturing the differential signaltransmission cable using the manufacturing system 201 begins with thedry-ice-blasting on the outer peripheral surface of the insulationlayer, followed by the corona discharge exposure process on the outerperipheral surface of the insulation layer, and then a permanganatetreatment. Then, after the permanganate treatment, the plating layer isformed on the outer peripheral surface of the insulation layer. Thedry-ice-blasting corresponds to the surface roughening treatment. Thecorona discharge exposure process corresponds to the surfacemodification treatment. The permanganate treatment facilitates platingon the insulation layer. The permanganate treatment also improves atransmission property of the differential signal transmission cable. Thepermanganate treatment may be conducted after the surface rougheningtreatment, and the corona discharge exposure process may be conductedthereafter.

The surface improving unit 203 may be configured as a surface improvingunit 203′ shown in FIG. 17. This surface improving unit 203′ comprises ahydrophilic treatment device 207′ in a cylindrical shape. Thehydrophilic treatment device 207′ comprises a shaft hole 217. Thedifferential signal transmission cable 171 conveyed by the bobbins 209,213 passes through the shaft hole 217. The hydrophilic treatment device207′ generates the corona discharge inside the shaft hole 217. Anexposure to the corona discharge causes hydrophilization of the outerperipheral surface of the insulation layer and improves the wettability.This decreases the contact angle and increases the absolute value of theadhesion-wetting surface free energy. The corona discharge exposureprocess corresponds to the surface modification treatment.

The surface improving unit 203 may be configured as a surface improvingunit 203″ shown in FIG. 18. A hydrophilic treatment device 207″ of thesurface improving unit 203″ comprises an arc-shaped first electrode 219a that faces the bobbin 213, and an arc-shaped second electrode 219 bthat faces the bobbin 215. The bobbins 213 and 215 are grounded to theearth. The hydrophilic treatment device 207″ generates the coronadischarge by applying a voltage between the first electrode 219 a andthe bobbin 213, and between the second electrode 219 b and the bobbin215. Exposure to the corona discharge hydrophilizes the outer peripheralsurface of the insulation layer and improves its wettability. Thisdecreases the contact angle and increases the absolute value of theadhesion-wetting surface free energy. The corona discharge exposureprocess corresponds to the surface modification treatment.

Signal transmission cables other than the differential signaltransmission cable can be manufactured by methods similar to the methodsas mentioned above.

3. Multicore Cable

A multicore cable in the present disclosure comprises signaltransmission cables, a conductor layer, and a jacket. The conductorlayer is configured to cover the signal transmission cablescollectively. The jacket is configured to cover the conductor layer.Each of the signal transmission cables has basically the same structureas the signal transmission cable explained in the aforementioned sectionof “1. Signal Transmission Cable” and further comprises an outerinsulation layer that is configured to cover the plating layer.

The signal transmission cables may be twisted or may not be twisted. Thenumber of the signal transmission cables is not particularly limited andmay be two, eight, or twenty-four, for example. For example, the signaltransmission cables may be divided into two or more groups, and aninterposition may be placed between the groups. Each group comprises,for example, two or more signal transmission cables. Each signaltransmission cable may be a differential signal transmission cable thatcomprises two signal lines or may be other type of signal transmissioncable.

The conductor layer may be constituted by, for example, a shield tapeconductor, and a braided wire. The conductor layer may comprise, forexample, stacked layers of the shield tape conductor and the braidedwire. Materials generally used for manufacturing cables can be used asmaterials for the shield tape conductor and the braided wire. Likewise,materials generally used for cables can also be used as materials forthe jacket.

An outer insulating layer may be, for example, an insulating tape, alaminating tape, a film with spray-applied insulator or the like. Thelaminating tapes such as those generally used for flat cables may beused, for example. The outer insulating layers should be preferablyformable at room temperature or at low temperature. This can reducedeformation of the insulation layer due to heat when forming the outerinsulating layer.

A material for the interposition may comprise, for example, a paper, athread, a foamed body and the like. Examples of the foamed body maycomprise polyolefin foams, such as a polypropylene foam and an ethylenefoam. The arithmetic average roughness Ra of the outer peripheralsurface of the insulation layer is 0.6 μm or greater in the signaltransmission cables of the multicore cable in the present disclosure.This results in high adhesion between the insulation layer and theplating layer, which then reduces occurrence of a gap between theinsulation layer and the plating layer. As a consequence, a highshielding effect by the plating layer is provided. At the same time,transmission loss can be reduced since the arithmetic average roughnessRa of the outer peripheral surface of the insulation layer is 10 μm orless.

FIG. 19 shows an example of a multicore cable 301. The multicore cable301 comprises eight differential signal transmission cables 302(comprising two transmission cables 302 a and six transmission cables302 b), a shield tape conductor 303, a braided wire 305, and a jacket307. The shield tape conductor 303 and the braided wire 305 both coverthe eight differential signal transmission cables 302 collectively. Thebraided wire 305 is situated around the outer circumference of theshield tape conductor 303. The jacket 307 covers the braided wire 305.

The eight differential signal transmission cables 302 are divided intotwo groups. The first group is in a central area and comprises twodifferential signal transmission cables 302 a and the second groupcomprises six differential signal transmission cables 302 b arrangedaround the first group. An interposition 309 is arranged between thefirst group and the second group.

FIG. 20 shows a configuration of each of the eight differential signaltransmission cables 302. Each differential signal transmission cable 302comprises the first and second signal lines 3 a and 3 b, the insulationlayer 5, the plating layer 7, and the outer insulation layer 311.

The insulation layer 5 covers the first and second signal lines 3 a and3 b collectively. The plating layer 7 covers the insulation layer 5. Theouter insulation layer 311 covers the plating layer 7. The maximum valueof the differential-to-common mode conversion energy of the differentialsignal transmission cable 302 is −26 dB or less in the frequency rangeof 50 GHz and below. The arithmetic average roughness Ra of the outerperipheral surface of the insulation layer 5 is between 0.6 μm and 10 μminclusive. The configuration of the first and second signal lines 3 aand 3 b, the insulation layer 5, and the plating layer 7 are the same asthose configurations explained in the previous sections, for example,“1. Signal Transmission Cable”.

4. Embodiments

(4-1) First Embodiment

A differential signal transmission cable 1 according to a firstembodiment comprising a configuration shown in FIG. 1 is manufactured.The material for the insulation layer 5 is polyethylene. The insulationlayer 5 covers the first and second signal lines 3 a and 3 bcollectively. The shape of an outer periphery of the insulation layer 5is elliptical in a section orthogonal to an axis of extention of thefirst and second signal lines 3 a and 3 b. The thickness of the platinglayer 7 is 4.56 μm. The standard deviation of the thickness of theplating layer 7 is 0.68 μm. The coefficient of variation of thethickness of the plating layer 7 is 0.15.

As shown in FIG. 20, an outer diameter of the insulation layer 5 alongthe major axis of the elliptic shape is denoted by L₁; an outer diameterof the insulation layer 5 along the minor axis of the elliptic shape isdenoted by L₂; and the distance between the center of the first signalline 3 a and the center of the second signal line 3 b is denoted by L₃.Along the major axis, the distance between the center of the secondsignal line 3 b and the outer peripheral surface of the insulation layer5, measured in a direction away from the first signal line 3 a withoutcrossing the first signal line 3 a, is denoted by L₄. The distancebetween the center of the first signal line 3 a and the outer peripheralsurface of the insulation layer 5, measured likewise, is also L₄. Themaximum distance between a straight line running across the centers ofthe first and second signal lines 3 a and 3 b and the outer peripheralsurface of the insulation layer 5 measured along the minor axis isdenoted with L₅.

Similarly, L₁ to L₅ can be defined in a case where the shape of theouter periphery of the insulation layer 5 is oval. In this case, themajor axis is parallel to two straight lines of the outer peripheralsurface of the insulation layer 5. In this case, the minor axis isorthogonal to the aforementioned two straight lines.

In the first embodiment, L₁ is 2.03 mm, L2 is 1.04 mm; L3 is 0.55 mm; L₄is 0.74 mm; and L₅ is 0.52 mm.

The surface roughening treatment was applied to the outer peripheralsurface of the insulation layer 5. Chromic acid etching was conducted asthe surface roughening treatment. The arithmetic average roughness Ra ofthe outer peripheral surface of the insulation layer 5 was 0.6 μm beforeforming the plating layer 7. The contact angle of the outer peripheralsurface of the insulation layer 5 was 95° before forming the platinglayer 7.

The differential-to-common mode conversion energy of the differentialsignal transmission cable 1 according to the first embodiment wasmeasured. The differential-to-common mode conversion energy was measuredbefore winding the differential signal transmission cable around a drumor the like. FIG. 21 shows the result of the measurement with thereference numeral “131”. In FIG. 21, the horizontal axis representsfrequency in logarithmic scale, and the vertical axis represents thedifferential-to-common mode conversion energy in decibels (dB). Thedifferential-to-common mode conversion energy in the vertical axiscorresponds to Scd21 in mixed-mode S-parameters. The larger the value onthe vertical axis is (that is, the smaller the absolute value of anegative measured value is), the greater the amount of noise in thedifferential-to-common mode conversion energy is. That is, a large valueon the vertical axis indicates that decrease of the quality of thetransmitted signal is significant.

In addition, the differential-to-common mode conversion energy was alsomeasured with respect to a differential signal transmission cable R inthe comparative example. FIG. 21 shows the result of the measurementwith the reference numeral “132”. The surface modification treatment wasnot conducted on the outer peripheral surface of the insulation layer ofthe differential signal transmission cable R in the comparative example.Therefore, the arithmetic average roughness Ra of the outer peripheralsurface of the insulation layer was 0.13 μm, and the contact angle ofthe outer peripheral surface of the insulation layer was 82°. Thedifferential signal transmission cable R in the comparative example doesnot comprise a plating layer, but comprises a conductor layer of a woundmetallic tape.

The differential-to-common mode conversion energy was small in thedifferential signal transmission cable 1 of the first embodimentcompared with that of the differential signal transmission cable R ofthe comparative example. Significant difference was seen particularly ina high frequency range.

The transmission property was measured with respect to the differentialsignal transmission cable 1 of the first embodiment and the differentialsignal transmission cable R of the comparative example. The transmissionproperty was measured before winding the differential signaltransmission cable around a drum or the like. FIG. 22 shows the resultof the measurement of the differential signal transmission cable 1 ofthe first embodiment with reference numeral “51”, and the result of themeasurement of the differential signal transmission cable R of thecomparative example with reference numeral “52”. The horizontal axis inFIG. 22 represents frequency of the transmission signal.

The vertical axis represents signal transmission loss in dB. Thetransmission loss in the vertical axis corresponds to Sdd21 of themixed-mode S-parameters. FIG. 22 shows that, as the value on thevertical axis decreases (that is, as the absolute value of the negativemeasured value increases), the attenuation of the transmission signalincreases, the degradation due to transmission of signal increases, andthe transmission loss becomes more significant.

The transmission loss of the differential signal transmission cable 1 ofthe first embodiment was less than that of the differential signaltransmission cable R of the comparative example. In addition, nosuck-out occurred in the differential signal transmission cable 1 of thefirst embodiment. Although it is not shown in FIG. 22, no suck-outoccurred also in the range of between 30 GHz and 50 GHz inclusive. Thesuck-out is a sudden attenuation of transmission signals.

In contrast, the suck-out occurred in the differential signaltransmission cable R of the comparative example. A mechanism that nosuck-out occurred in the differential signal transmission cable 1 of thefirst embodiment is assumed that the plating layer is formedcontinuously across the entire surface of the differential signaltransmission cable 1, causing no overlapping layer or seam that would beseen on the conductor layer of a wound metallic tape.

(4-2) Second Embodiment

The differential signal transmission cables S1 to S7 were manufacturedunder the conditions shown in Table 4.

TABLE 4 Sample Permanganate First Ra Second Ra Average Ra 12.89 GHz No.Pre-treatment treatment (μm) (μm) (μm) Sdd21 S1 Dry-ice Applied 2.973.56 3.27 −8.15 S2 blasting + Applied 3.23 1.49 2.36 −8.14 S3 CoronaApplied 0.93 1.04 0.99 −7.91 S4 discharge Not applied 4.22 3.34 3.78−8.65 S5 Not applied 1.33 1.20 1.27 −8.32 S6 Chromic acid — — — 0.60−7.80 treatment S7 Cu taping — — — 0.08 −7.50 (lateral winding)

S1 to S7 each comprises the polyethylene insulation layer. In each of S1to S7, the shape of the outer periphery of the insulation layer iselliptical in a section orthogonal to an axis of extension of the firstand second signal lines 3 a and 3 b. In the second embodiment, L₁ is1.21 mm; L2 is 0.62 mm; L3 is 0.35 mm; L₄ is 0.43 mm; and L₅ is 0.31 mm.

S1 to S6 each comprises an electrically conducting layer constituted bythe plating layer. S7 comprises an electrically conducting layer formedby laterally winding a Cu tape. In S1 to S5, the outer peripheralsurface of the insulation layer was processed by the dry-ice-blasting,followed by the corona discharge exposure process. The dry-ice-blastingcorresponds to the surface roughening treatment. The corona dischargeexposure process corresponds to the surface modification treatment. InS1 to S3, the permanganate treatment was conducted after the coronadischarge exposure process. In S4 and S5, the permanganate treatment wasnot conducted after the corona discharge exposure process. In S6,chromic acid treatment was conducted on the outer peripheral surface ofthe insulation layer. In S7, no pre-treatment was conducted prior towinding the Cu tape.

Regarding S1 to S7, the arithmetic average roughness Ra and thetransmission loss Sdd21 were measured. Table 4 shows the results of themeasurement. In Table 4, “First Ra” represents the arithmetic averageroughness Ra at the first measuring position; “Second Ra” represents thearithmetic average roughness Ra at the second measuring position; and“Average Ra” represents the average value of First Ra and Second Ra.FIG. 23 shows the relation between the arithmetic average roughness Raand the transmission loss Sdd21. The transmission loss Sdd21 decreasesas the arithmetic average roughness Ra decreases. S1 to S3 applied withthe permanganate treatment had less transmission loss than S4 to S5applied with no permanganate treatment.

5. Other Embodiments

The embodiments of the present disclosure have been explained above. Thepresent disclosure may be achieved in various modifications withoutbeing limited to the explained embodiments.

(1) In each of the aforementioned embodiments, one function of oneelement may be achieved by two or more elements, or two or morefunctions of two or more elements may be achieved by one element. A partof the configuration of the aforementioned embodiments may be omitted,and at least a part of the configuration of the aforementionedembodiments may be added to or replaced with another part of theaforementioned embodiments. It should be noted that any and all modesencompassed in the technical ideas that are defined by the languages inthe claims are embodiments of the present disclosure.

(2) In addition to the aforementioned signal transmission cable andmulticore cable, the present disclosure may also be achieved in variousforms, such as a system comprising at least one of the aforementionedsignal transmission cable or multicore cable, a manufacturing method ofthe multicore cable, a method of signal transmission and reception usingthe signal transmission cable.

What is claimed is:
 1. A signal transmission cable comprising: a signalline; an insulation layer configured to cover the signal line; and aplating layer configured to cover the insulation layer, wherein anarithmetic average roughness Ra of an outer peripheral surface of theinsulation layer is between 0.6 μm and 10 μm inclusive.
 2. The signaltransmission cable according to claim 1, wherein a thickness of theplating layer is between 1 μm and 5 μm inclusive, wherein a standarddeviation of the thickness of the plating layer is 0.8 μm or less, andwherein the standard deviation of the thickness of the plating layer ismeasured by taking four cross sections of the signal line orthogonallyto an axis of extension of the signal line, taking four random points oneach cross section to make sixteen points in total, and measuring athickness of the plating layer at each of the sixteen points and obtaina standard deviation, which is used as the standard deviation of thethickness of the plating layer.
 3. The signal transmission cableaccording to claim 1, wherein a contact angle on the outer peripheralsurface of the insulation layer is 95° or less.
 4. The signaltransmission cable according to claim 1, wherein an absolute value ofadhesion-wetting surface free energy on the outer peripheral surface ofthe insulation layer is 66 mJ/m² or greater.
 5. The signal transmissioncable according to claim 1, wherein the outer peripheral surface of theinsulation layer comprises a concavity, and wherein the concavitycomprises, at its bottom in a depth direction, a space that is widerthan an opening of the concavity.
 6. The signal transmission cableaccording to claim 1, wherein the insulation layer is made ofpolyethylene, wherein a crystallinity X_(c) expressed in followingFormula 1 is 0.744 or greater, and $\begin{matrix}{X_{c} = \frac{I_{c}}{I_{c} + I_{a}}} & \left\lbrack {{Formula}\mspace{14mu} 1} \right\rbrack\end{matrix}$ wherein I_(c) in the Formula 1 is X-ray diffractionintensity of a crystalline component, and I_(a) is X-ray diffractionintensity of a noncrystalline component.
 7. The signal transmissioncable according to claim 1, wherein the insulation layer is made ofperfluoroethylene-propene copolymer, wherein a crystallinity X_(c)expressed in the following Formula 1 is 0.47 or less, and$\begin{matrix}{X_{c} = \frac{I_{c}}{I_{c} + I_{a}}} & \left\lbrack {{Formula}\mspace{14mu} 1} \right\rbrack\end{matrix}$ wherein I_(c) in the Formula 1 is X-ray diffractionintensity of a crystalline component, and I_(a) is X-ray diffractionintensity of a noncrystalline component.
 8. The signal transmissioncable according to claim 1, wherein the insulation layer is made ofpolyethylene, wherein the polyethylene has crystal structures of thetriclinic crystal system or of the orthorhombic system, or has acoexisting state of at least one of these crystal structures, whereinthe polyethylene has preferential crystalline orientations in specifictwo or less number of crystal axes, wherein a (100) crystal orientationdegree O₁₀₀ of the polyethylene expressed in the following Formula 2 is0.26 or less, and $\begin{matrix}{O_{100} = \frac{I_{200}}{I_{110} + I_{200}}} & \left\lbrack {{Formula}\mspace{14mu} 2} \right\rbrack\end{matrix}$ wherein I₂₀₀ in the Formula 2 is X-ray diffractionintensity of the index 200, and I₁₁₀ is X-ray diffraction intensity ofthe index
 110. 9. The signal transmission cable according to claim 1,wherein the insulation layer is made of polyethylene, and wherein thepolyethylene has a crystalline size of 18 nm or greater in a crystallinecomponent.
 10. The signal transmission cable according to claim 1,wherein the insulation layer is made of perfluoroethylene-propenecopolymer, and wherein the perfluoroethylene-propene copolymer has acrystalline size of 13.6 nm or less in a crystalline component.
 11. Amulticore cable comprising: signal transmission cables; a conductorlayer configured to cover the signal transmission cables collectively;and a jacket configured to cover the conductor layer, wherein each ofthe signal transmission cables comprises the signal transmission cableaccording to claim 1 and an outer insulation layer configured to coverthe plating layer.
 12. A method of manufacturing a signal transmissioncable comprising a signal line, an insulation layer configured to coverthe signal line, and a plating layer configured to cover the insulationlayer, the method comprising: covering the signal line with theinsulation layer, followed by conducting dry-ice-blasting on an outerperipheral surface of the insulation layer, followed by conducting acorona discharge exposure process on the outer peripheral surface, andforming the plating layer on the outer peripheral surface.
 13. Themethod of manufacturing the signal transmission cable according to claim12, wherein an arithmetic average roughness Ra of the outer peripheralsurface of the insulation layer is between 0.6 μm and 10 μm inclusiveafter the dry-ice-blasting is conducted.